Exxon Mobil Corp is working on a plan to repair and restart a ruptured Montana pipeline that spilled up to 1,000 barrels of crude into the Yellowstone River last weekend, but restoration is not expected for at least two weeks, an executive said on Wednesday.

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"Restoration of the line is something we'll look at separately," said Gary Pruessing, president of Exxon Mobil Pipeline Company. "It's not something that's going to happen in the next day, or week, or couple of weeks."

He said the immediate concern is cleanup of leaked oil from the Silvertip line.

The U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) and the Environmental Protection Agency are investigating the cause of the spill, along with Exxon Mobil and Montana state environmental regulators.

Pruessing also said Exxon continues to estimate that 750 to 1,000 barrels of oil leaked into the river, but the company's "best engineering estimate is the 750 number."

He said the company also calculated a worse outcome to come up with the 1,000-barrel figure "to make sure we did not underestimate the impact that we're having to deal with."

Pruessing also addressed questions over how long it took Exxon to shut down the pipeline once the leak was discovered late Friday.

Exxon shut down pumps in the 70-mile pipeline that supplies its 60,000 barrel-per-day (bpd) refinery in Billings, Montana, within seven minutes of discovering a drop in pressure, Pruessing said.

Photo shows oil mixed with water from an oil spill along the Yellowstone River in Laurel, Montana, July 6, 2011. Credit: Reuters/John Warner